In the drilling of boreholes, it is necessary to include a rotating blowout preventor at the upper end of the surface casing. The rotating blowout preventor sealingly receives the kelly in a slidable manner so that the kelly can support and drive the drill string while at the same time fluid is conveyed downhole to the drill bit, back up through the borehole annulus, and back out of the rotating blowout preventor. In the event a pay zone is encountered, the rotating blowout preventor restrains the increased pressures and maintains the borehole under control, thereby precluding a "blowout" occurring.
In the drilling of boreholes, it is not unusual for the stripper rubber of the rotating blowout preventor to become worn and require replacement several times during the formation of a single borehole. It would be advantageous to be able to provide a rotating blowout preventor fabricated in such a manner that it could endure for the interval of time required in making one complete borehole.
In my previous patent application Ser. No. 843,291; filed Oct. 18, 1977; now U.S. Pat. No. 4,154,448; issued May 15, 1979, there is set forth a rotating blowout preventor having an improved stripper rubber which is reinforced in such a manner that the stripper rubber engages the kelly with sufficient force to enable the stripper to rotate the rotating components of the rotating blowout preventor.
In many instances, it is desirable to alleviate the rotational load or torque imparted by the kelly into the stripper rubber by the provision of a kelly drive bushing which slidably receives the kelly to cause the edges of the kelly to engage and drive the rotating components of the rotating blowout preventor, thereby reducing the torque imparted into the rubber. When a kelly drive bushing is incorporated into the rotating blowout preventor of the present invention, it is necessary to index the reinforcing fingers of the stripper rubber with the kelly drive bushing so that as the kelly is received through the axial passageway of the rotating blowout preventor, the corners of the kelly simultaneously engage both the kelly drive bushing and the elastomer of the stripper rubber such that the corners of the kelly are simultaneously positioned at the appropriate location on both the kelly drive bushing and the stripper rubber. Such an improvement in a rotating blowout preventor provides unusually good sealing characteristics of the stripper rubber respective to the kelly as well as greatly elongating the life of the stripper rubber.